In order for post-combustion carbon dioxide capture technology to realize widespread viability, the energy cost of this technology must be drastically reduced. Current adsorbent technologies that rely on pressure, temperature or vacuum swing adsorption consume as much as 40% of the power plant's production capacity, most of which is associated with the liberation of the CO2 from the capture medium. Ultimately this penalty, or parasitic energy load, must be brought closer to the thermodynamic minimum of about 4% to avoid prohibitive cost increases. Given that the triggers for release of adsorbed carbon dioxide are so energy intensive and are based on energy from the power plant, there is strong motivation to develop new, low energy release triggers, utilising renewable energy sources. In conjunction with this, adsorbents with maximum performance can further reduce the cost compared to the conventional energy intensive CO2 gas separation process.
A range of different types of materials have been considered for use in separation materials for the separation of selected gases, and notably CO2 from a gas stream. Materials include porous organic polymers and metal-Organic Frameworks (MOFs), amongst others. MOFs are an important class of 3D crystalline porous materials comprised of metal centres and organic ligands, joined periodically to establish a crystalline porous array. The large internal surface areas can be used to adsorb large quantities of gases, such as hydrogen, methane and carbon dioxide.
Methods for the incorporation of light responsive groups within MOFs include use of pendant groups pointing into the pores, and filling of pores with light responsive guest molecules. The responsive groups within these materials may then change their conformation when exposed to filtered light which results in a change in adsorption capacity (in static conditions). Whilst these initial results are exciting, there are inherent limitations in the approaches reported to date. Firstly there is a requirement for specific wavelengths of light to trigger the conformational change. Second, the mode of regeneration in materials studied to date has involved mechanisms that take considerable time to achieve removal of the adsorbed species. Some mechanisms require the application of considerable energy in the form of heat.
An adsorbent that can respond to a broad light spectrum similar to solar radiation, and/or possess relatively fast photo-switching that directly releases CO2 would offer enhanced, lower energy routes to light-triggered CO2 release.